Method of manufacture of a structure and a structural member for use in the method

ABSTRACT

An automobile structure includes a first extruded structural member with longitudinally extending ribs on an exterior surface, and a second structural member joined to the first structural member with adhesive, wherein the depth of the adhesive between the first and second structural members varies with variations in the cross-section of the first structural member, and the ribs define a gap of a specified depth between facing surfaces of the two joined structural members to enhance the performance of the adhesive bond. In the method, one of the first and second structural members is coated with an excess of adhesive in a desired region, the first and second structural members are clamped tightly together using mechanical fastening means while the adhesive remains fluid, the tips of the ribs of the first structural member directly abut an adjacent surface of the second structural member, excess adhesive is disposed along the channels defined between the ribs, and the mechanical fastening means reinforces the formed joint.

[0001] The present invention relates to a method of manufacture of astructure and to a structural member for use in the method. Theinvention particularly relates to the manufacture of a space framestructure for an automobile, but should not be considered limited tosuch an application.

[0002] Recently there has been interest in reducing the weight ofautomobiles in order to improve fuel economy and performance. There hasconsequently been a move to use aluminium in place of steel in themanufacture of automobiles. Whilst aluminium has many advantages oversteel, in particular in terms of weight saving, it is difficult to forma space frame for a vehicle from aluminium since welding of aluminium isdifficult.

[0003] U.S. Pat. No. 5,209,541 discloses a method of manufacture of aspace frame for an automobile from aluminium structural members. Themethod involves the bonding of the aluminium structural members togetherwith a suitable bonding material such as an acrylic adhesive. The methodrequires that two structural members to be joined together are eachprovided with a shaped channel and that an interconnecting member isprovided to be inserted into both the channels of the two aluminiummembers, with the interconnecting member being secured there byadhesive. The method involves complications since the structural membersmust be machined to give a specific cross section in the areas of jointsand also since interconnectors must be used.

[0004] DE-A-3525830 shows how a bootlid for an automobile can be made bythe bonding together of two metal sheets which are shaped with grooveswhich provide a gap for adhesive.

[0005] DE-A-3811427 shows how two tubular members of an automobile spaceframe can be joined together end to end by providing the tubular memberswith specifically manufactured end portions which cooperate together todefine a series of channels for receiving adhesive and also twoapertures through which adhesive can be injected in order to adhere thetwo components together.

[0006] EP-A-0523831 shows how a motor car structure can be built up fromflat panels joined together, each panel comprising two mutually parallelspaced apart structural skins braced with respect to each other by acore of expanded material united to both skins. Each joint between thetwo panels comprises a series of integral projections formed on a firstpanel formed by cutting the first panel to a required shape and alsorecesses in the second panel which correspond to the projections, theprojections being inserted into and bonded in the recesses by means ofan adhesive.

[0007] DE-A-4202391 shows how two automobile panels can be connectedtogether by a joint with an L-shaped cross section, which acts as aninterconnecting member. The L-shaped joint has two channels, one foreach of the panels to which it is joined. Each channel is formed with across section which defines a groove for receiving adhesive in orderthat a panel inserted into the channel can be adhered to the channel byadhesive in the groove.

[0008] DE-A-4124627 shows how a body panel for a vehicle can be providedwith a corrugated edge section so that the panel can be joined toanother panel by inserting the corrugated edge into a channel providedat the edge of the second panel. The corrugated edge serves to locatethe first panel in the channel at the edge of the second panel whilstthe adhesive is setting and also serves to define a gap for theadhesive.

[0009] The present invention in a first aspect provides a method ofmanufacture of a structure which comprises a plurality of structuralmembers bonded together, the method comprising the steps of extruding astructural member with a chosen cross-section, and joining extrudedstructural member to another structural member by bonding with anadhesive, wherein when the extruded structural member is bonded to thestructural member the depth of the adhesive between the two structuralmembers varies with variations in the cross-section of the extrudedstructural member, characterised in that the cross-section of theextruded structural member is chosen so as to enhance the performance ofthe adhesive bond between the structural members.

[0010] In a second aspect the present invention provides a structuralmember for use in the method of manufacture described above which is ahollow, elongate extrusion of aluminium or an alloy of aluminium andwhich has a plurality of longitudinally extending spaced apart ribs fordefining a gap between the structural member and a further structuralmember bonded thereto.

[0011] In a third aspect the present invention provides a method ofmanufacture of a structure wherein first and second structural membersare bonded together with adhesive, characterised in that the firststructural member is provided on a first surface with a plurality ofspaced apart exterior ribs which abut an adjacent surface of the secondstructural member when the structural members are bonded together andwhich define between the first surface of the first structural memberand the adjacent surface of the second structural member a gap forreceiving adhesive, whereby the ribs define a chosen depth of adhesivebetween the first and second structural members in order to optimise thebond between the first and second structural members.

[0012] In a fourth aspect the present invention provides a structuralmember for use in the above described method which is an extrusion andwhich has a plurality of longitudinally extending exterior ribs providedon a first surface to define between the first surface and an adjacentsurface of a second structural member bonded to the structural member agap for receiving adhesive, whereby the ribs can define a chosen depthof adhesive.

[0013] In a further aspect of the present invention there is provided amethod of manufacture of a structure which comprises a tubular elongatestructural member, the method comprising the steps of forming thetubular elongate structural member with a chosen cross-section, andjoining the tubular elongate structural member to a second structuralmember by bonding with an adhesive, wherein when the tubular elongatestructural member is bonded to the second structural member the depth ofthe adhesive between facing surfaces of the tubular elongate structuralmember and the second structural member varies with variations in thecross-section of the tubular elongate structural member, characterisedin that the cross-section of the tubular elongate structural member ischosen to enhance the performance of the adhesive bond between thetubular elongate structural member and the second structural member.

[0014] Bonding of structural members with adhesive and optionally withmechanical fasteners in addition to adhesive, is advantageous overwelding and over the use of mechanical fasteners alone. The resultingbond does not suffer from heat distortion and does not suffer fromreduction in material properties occasioned by the heat of welding. Theresulting joint is also stiffer that a joint which would result frompurely mechanical fastening (e.g. with nuts and bolts). Furthermore, thebonding process enables the use of hollow tubular members of a wallthickness thinner than that which would be necessary if welding is usedas a joining method (welding causes some depletion of metal andconsequent weakening of the structural members). However, the strength,efficiency and durability of a bonded joint depends on the thickness ofadhesive between the two structural elements joined. Therefore it isvery important to control the thickness of the adhesive layer betweenthe two structural members. The present invention solves this problem inall of its aspects by shaping the members to be joined in a fashion thatthe depth of adhesive is a planned and controlled depth. The extrusionof material to form a structural member with a chosen cross-section is avery efficient way of forming the structural member, particularly if themember is made of aluminium or an alloy of aluminium. The requiredcross-section is obtained in one operation and the formed memberrequires little or no further machining.

[0015] Preferred embodiments of the present invention will now bedescribed with reference to the accompanying drawings in which:

[0016]FIG. 1 is a drawing illustrating a typical joint in a structureaccording to the present invention.

[0017]FIG. 2 is a cross-section through the joint illustrated in FIG. 1.

[0018]FIG. 3 is a figure which illustrates the distribution of stressthrough a prior art joint.

[0019]FIG. 4 is an illustration of a second type of joint in a structureaccording to the present invention.

[0020] In FIG. 1 a first hollow extruded aluminium structural member 10is joined to a second hollow extruded aluminium structural member 11 bybonding. This figure shows a joint between two structural members in aspace frame for an automobile. It will be understood that the totalspace frame will comprise a number of different aluminium structuralmembers, all joined together to form the required space frame shape.FIG. 1 shows only a joint between two such extruded hollow aluminiumstructural members, but it should be understood that the figure isrepresentative of how the extruded hollow aluminium structural membersare joined throughout the space frame.

[0021] The hollow extruded aluminium structural member 10 is formed witha constant cross-section of a generally rectangular form. Four planarregions are provided on the exterior of the structural member 10, eachplanar surface being orthogonal to its two nearest planar surfaces andjoined thereto by curved regions of the structural member 10 whichdefine the four corners of a cross-section through the structuralmember. The structural member 10 is formed by an extrusion process,aluminium being particularly suited to an extrusion type ofmanufacturing. Manufacturing by extrusion is a particularly useful andcost effective method for low volume manufacture of space frames.

[0022] In the past, space frame manufacturers have bonded a hollowextruded aluminium structural member such as member 10 directly to amember of identical cross-section, with two planar surfaces beingbrought together within a layer of adhesive between the surfaces. Thisis illustrated in FIG. 3. The disadvantage of doing this arises from thefact that bond thickness in a joint is a very important factor indetermining the strength efficiency and durability of the bonded joint.When two planar surfaces are brought together, it is difficult tocontrol the bond thickness between the surfaces and therefore theresulting joints are very variable in their strength efficiency anddurability. The present invention overcomes this problem bymanufacturing the second structural element 11 with a specially chosencross-section.

[0023] The structural member 11 is manufactured in the preferredembodiment from aluminium using an extrusion process. The structuralelement is provided with small ribs 12 on one exterior surface of aplanar region of the structural member. Since the structural member ismanufactured by extrusion, it is a very easy, low cost process toprovide the small ribs 12. Providing the small ribs 12 by any othermanufacturing process would be difficult to do cost effectively.However, extrusion is a very good process for providing a member with aconsistent cross-section manufactured to high tolerances which does notrequire any general further machining in order to bring it into a formsuitable for use. Also, extruded sections tend to have a very goodsurface finish, and generally do not require further operations in orderto bring the surface finish to an acceptable quality.

[0024] As can be seen clearly in FIGS. 1 and 2, the extruded aluminiumstructural member 11 is provided with a plurality of parallel ribs 12which extend along the length of the elongate structural member 11. Whenthe elongate hollow aluminium extruded structural member 11 is to bebonded to the structural member 10 then the surface of the member 11 iscoated with adhesive in a desired region and then the two structuralmembers 10 and 11 are clamped tightly together by an assembly fixturewhile the adhesive sets. Alternatively, it may be desired that theformed joint is reinforced by means of mechanical fastenings, such asnuts and bolts, rivets or screws. It is particularly preferred thatEJOTS screws are used since these are easy to use and a high tear outforce must be applied to loosen them. In this case, the structuralmembers 10 and 11 are bolted or screwed or otherwise connected to eachother whilst the bonding adhesive remains fluid.

[0025] When the structural members 10 and 11 are mechanically clampedtogether, the top of the ribs 12 on the structural member 11 come intodirect abutment with the adjacent planar surface of the structuralmember 10. Thus, this ribs 12 define a bond thickness over almost theentire area of the joint. A bond thickness of 0.2 mm is the optimal bondthickness for an epoxy adhesive. obviously, the ribs could be formedwith different heights for different types of adhesive in order alwaysprovide the optimal bond thickness. The longitudinal ribs 12 allowadhesive to flow along channels between the ribs 12 along the structuralmember 11 so that an excess of adhesive can be applied to the surface ofthe structural member 11 before the two structural members 10 and 11 arejoined, with the excess adhesive flowing along the channels when thestructural member 10 is brought into abutment with the top of the ribs12. Alternatively, it may be preferred to bring the structural member 10into abutment with the ribs 12 first and then use the defined channelsas channels into which bonding material can be injected.

[0026] Since the ribs 12 extend all along the elongate structural member11 the member 10 can be bonded to the member 11 at any point along itslength.

[0027] As mentioned above, FIG. 3 shows a typical bonded joint accordingto the prior art. In the original bonded joint, the stress distributionin the adhesive is uneven as illustrated by the graph of stressdistribution shown in FIG. 3 when a tangential force F is applied to themember 10 in the direction indicated by the arrow in FIG. 3. The stressin the adhesive is very much greater at the edges of the bond areas thatin the rest of the area; this makes the bond susceptible to peel. Thisis clearly unsatisfactory as optimum strength of a bond is achieved onlyif the adhesive is stressed to the same level over the joint. When theprior art joint is highly loaded then the adhesive begins cracking atthe lines of highest stress, whilst the stress in the rest of theadhesive is much lower. The cracks can initiate failure of the joint atloads significantly below the optimum load which the joint should becapable of carrying.

[0028] The present invention deals with the problem of uneven stressloading by providing one of the structural members to be joined with across-section which acts to define a varying adhesive depth across thejoint, the varying adhesive depth acting to optimise the strength of thejoint. FIG. 4 shows this by showing how two structural members 20 and 21can together define a varying bond thickness. In FIG. 4 three differentdistinct regions can be seen in the joint. Area Q-R has the optimalsmall thickness of adhesive (e.g. 0.2 mm for an epoxy adhesive). Thethickness of adhesive in this region is preferably controlled asdescribed above, by the use of ribs (although these are not strictlynecessary and therefore are not shown in FIG. 4). Areas P-Q and R-S areareas of increasing thickness. The profile of the structural member 20is defined in order to provide the variation of bond thickness required,the cross-section of the structural member 20 being defined simply andcheaply by an extrusion process. The profile of the structural member 20in the areas P-Q and R-S may be straight, convex or concave according tothe requirements of the particular adhesive used in the bonds.

[0029] By defining thicker areas of adhesive at the edges of the jointthe performance of the joint is improved. This arises since the areas ofgreater thickness towards the edges of the bond area are less stiff thanthe area of bond in the region Q-R and therefore when the joint isloaded tangentially by a force F in the direction shown in FIG. 4 thestrain imparted on the edges of the bond will be determined by theoverall stiffness of the joint in reaction to the imposed load, sincethe less stiff edge regions will deform to allow strain in the centralportion of the joint which thus shares the loading. Thus, an imposedload will result in a lower level of stress in the adhesive at the edgesof the bonded joint and the performance of the joint as a whole isimproved.

[0030] A joint between the two structural members 20 and 21 is formed bycoating the structural member 21 with a layer of adhesive and thenbringing the structural member 20 into engagement with the adhesive. Thestructural members 20 and 21 could be mechanically clamped together(either by external clamps or by mechanical fasteners if the joint is tobe both mechanically fastened and bonded).

[0031] The curved edge profile shown in FIG. 4 (or indeed any curvededge profile) would be avoided in the prior art since square sharp edgessuch as shown in FIG. 4 are preferred for welding purposes. It isdifficult to weld a component with curved edges since the weldingprocess would lead to inclusion in the weld. For this reason squareedged sections have been preferred in the prior art. In contrast thepresent invention teaches the skilled man to design the edge profiles tomatch the stress distribution across the join.

[0032] In the above illustrated embodiments of the invention it will beappreciated that the cross-sections required are obtained by extrusionof aluminium to form a member with a required cross-section. Asmentioned above, extrusion makes cost effective forming of members withparticular cross-sections viable, since if an extruded section will haveclose tolerances and will require little in the way of materialtreatment after forming.

[0033] Whilst the invention as described in preferred embodiments isapplied to tubular elongate members for use in a space frame of anautomobile, it should be appreciated that the elongate structuralmembers can be used to form other structures. Also, the invention has awider scope in that it can also be applied to other sorts of structuralmembers such as panels, which can be provided with surface ribs or witha chosen cross-section in order provide beneficial bonding effects. Theinvention is less preferably applied to panels, since panels aredifficult to extrude and the invention is ideally suited to theextrusion process. The invention is, however, not limited in itsbroadest scope to extruded structural members, since, for instance, itis possible to make structural members which ribs defining bondthickness by other less preferable methods, e.g. rolling or casting.

[0034] The present invention becomes particularly important when highmodulus (high stiffness) adhesives are used since the detrimentaleffects of having a wrong thickness or a disadvantageous stressdistribution are enhanced.

[0035] It will be appreciated that many types of adhesive could be usedin the invention, but the preferred types are acrylic adhesives or epoxyadhesives.

1. A method of manufacture of a structure which comprises a plurality ofstructural members bonded together, the method comprising the steps ofextruding a first structural member with a chosen cross-section, andjoining the first structural member to a second structural member bybonding with an adhesive, wherein when the first structural member isbonded to the second structural member the depth of the adhesive betweenthe first and second structural members varies with variations in thecross-section of the first extruded structural member, characterised inthat the cross-section of the first extruded structural member is chosenso as to enhance the performance of the adhesive bond between the firstand second structural members.
 2. A method of manufacture as claimed inclaim 1 wherein the cross-section of the first extruded structuralmember is chosen such that the structural member has means to definebetween facing surfaces of the two bonded structural members a gap to befilled with adhesive.
 3. A method of manufacture as claimed in claim 2wherein the means defining the gap comprises a plurality of ribs onthe-exterior of the first extruded structural member.
 4. A method ofmanufacture as claimed in claim 3 wherein the adhesive used is an epoxyadhesive and the ribs define a gap with a depth in the range 0.15 to0.25 mm.
 5. A method of manufacture as claimed in claim 3 or 4 whereinthe first extruded structural member is a hollow aluminium elongatemember with four planar regions each of which is joined to two otherplanar surfaces by a curved corner region and a series of longitudinallyextending ribs provided externally on an external surface of one of theplanar regions.
 6. A method of manufacture as claimed in any one of thepreceding claims wherein the adhesive is spread over a bonding areabetween the first and second structural members and the cross-section ofthe first extruded structural member is chosen such that there is aregion of adhesive near an edge of the bonding area which is of greaterdepth than the adhesive at the centre of the bonding area.
 7. A methodof manufacture as claimed in claim 6 wherein the cross-section chosenfor the extruded structural member is chosen to define a region ofconstant adhesive depth in the centre of the bonding area and a regionof increasing adhesive depth extending from the central region ofconstant depth towards an edge of the bonding area.
 8. A method ofmanufacture as claimed in claim 7 wherein the first extruded structuralmember is a hollow aluminium elongate member with a transversecross-section having four generally planar regions and curved regionsjoining each two adjacent planar regions, two of the curved regions eachdefining a region of increasing adhesive depth when the first extrudedstructural member is joined to the second structural member.
 9. A methodof manufacture as claimed in claim 8 wherein the hollow aluminiumelongate member has a plurality of longitudinally extending exteriorribs which set the depth of the adhesive in the region of constantadhesive depth.
 10. A method of manufacture as claimed in any one of thepreceding claims wherein the structure manufactured by the method is aspace frame for an automobile.
 11. A structural member for use in themethod of manufacture of claim 5 or claim 9 which is a hollow, elongateextrusion of aluminium or an alloy of aluminium and which has aplurality of longitudinally extending spaced apart ribs for defining agap between the structural member and a further structural member bondedthereto.
 12. A structural member as claimed in claim 11 which has fourplanar regions and four curved regions, each curved region joiningtogether two planar regions, wherein the longitudinal extending ribs areprovided on an exterior surface of only one of the planar regions.
 13. Astructural member as claimed in claim 11 or claim 12 wherein thelongitudinally extending ribs extend over substantially the whole lengthof the first structural member and are parallel to one another.
 14. Amethod of manufacture of a structure wherein first and second structuralmembers are bonded together with adhesive, characterised in that thefirst structural member is provided on a first surface with a pluralityof spaced apart exterior ribs which abut an adjacent surface of thesecond structural member when the structural members are bonded togetherand which define between the first surface of the first structuralmember and the adjacent surface of the second structural member a gapfor receiving adhesive, whereby the ribs define a chosen depth ofadhesive between the first and second structural members in order tooptimise the bond between the first and second structural members.
 15. Amethod as claimed in claim 14 wherein the adhesive used is an epoxyadhesive and the chosen depth of adhesive is in the range 0.15 mm to0.25 mm.
 16. A method as claimed in claim 14 or claim 15 wherein thefirst structural member is composed of aluminium or an alloy ofaluminium.
 17. A method as claimed in any one of claims 14 to 16 whereinthe first structural member is an extrusion and the ribs are formedduring extrusion of the first structural member.
 18. A method as claimedin claim 17 wherein the first structural member is a hollow elongatemember.
 19. A method as claimed in any one of claims 14 to 18 whereinthe manufactured structure is a space frame for an automobile.
 20. Astructural member for use in the method of any of claims 14 to 19 whichis an extrusion and which has a plurality of longitudinally extendingexterior ribs provided on a first surface to define between the firstsurface and an adjacent surface of a second structural member bonded tothe structural member a gap for receiving adhesive, whereby the ribs candefine a chosen depth of adhesive.
 21. A structural member as claimed inclaim 20 which is composed of aluminium or an alloy of aluminium.
 22. Astructural member as claimed in claim 20 or claim 21 which is hollow andelongate.
 23. A structural member as claimed in claim 22 which has fourplanar regions and four curved regions, each curved region joiningtogether two planar regions, wherein the longitudinal ribs are providedon an exterior surface of one planar region.
 24. A structural member asclaimed in claim 22 or claim 23 wherein the ribs extend oversubstantially the entire length of elongate member in parallel spacedapart relationship.
 25. A method of manufacture of a structure whichcomprises a tubular elongate structural member, the method comprisingthe steps of forming the tubular elongate structural member with achosen cross-section, and joining the tubular elongate structural memberto a second structural member by bonding with an adhesive, wherein whenthe tubular elongate structural member is bonded to the secondstructural member the depth of the adhesive between facing surfaces ofthe tubular elongate structural member and the second structural membervaries with variations in the cross-section of the tubular elongatestructural member, characterised in that the cross-section of thetubular elongate structural member is chosen to enhance the performanceof the adhesive bond between the tubular elongate structural member andthe second structural member.
 26. A method of manufacture of a structureas claimed in claim 25 wherein the second structural member is also atubular elongate structural member which in the method is formed with across-section chosen to enhance the performance of the adhesive bondbetween the elongate tubular structural member and the second structuralmember.
 27. A method of manufacture of a structure as claimed in claim25 wherein the adhesive is spread over a bonding area between thetubular elongate structural member and the second structural member andthe cross-section of the tubular elongate structural member is chosensuch that there is a region of adhesive near an edge of the bonding areawhich is of greater depth than the adhesive at the centre of the bondingarea.
 28. A method of manufacture as claimed in claim 27 wherein thecross-section chosen for the tubular elongate structural member ischosen to define a region of constant adhesive depth in the centre ofthe bonding area and a region of increasing adhesive depth extendingfrom the central region of the constant depth towards an edge of bondingarea.
 29. A method of manufacture as claimed in any one of claims 26 to28 wherein the joint between the tubular elongate structural member hasa chosen strength and the tubular elongate structural member has a wallthickness chosen to be smaller than the wall thickness which would benecessary if the tubular elongate structural member were to be welded tothe second structural member with the chosen joint strength.
 30. Amethod of manufacture of a structure which has first and secondstructural members bonded together with adhesive, the method beingsubstantially as hereinbefore described with reference to FIGS. 1, 2 and4 of the accompanying drawings.
 31. A structural member for use in amethod of manufacture of a structure which has first and secondstructural members bonded together with adhesive, the structural memberbeing substantially as hereinbefore described with reference to FIGS. 1,2 and 4 of the accompanying drawings.